Patent GB-1258985-A (GB)
IPC: H04M3/58
Filed: 1969-09-04
Abstract: 1,258,985. Telephone exchanges. INTERNATIONAL STANDARD ELECTRIC CORP. 4 Sept., 1969 [6 Sept., 1968], No. 43794/69. Heading H4K. A private communication system comprises one main and a plurality of extension stations each selectively connectable to a common internal speech line, to a common exchange line and to a common call-back line associated with the exchange line, and each station has two keys, one of which when depressed connects that station to the exchange line and prepares the connection to the call-back line, and the other of which if subsequently operated connects the station to the call-back line. The Figure shows the main station RHSt, a control unit RSK and one substation RNSt2. More than one exchange line may be provided in which case corresponding call-back lines must be included.  Internal calls.-The stations are all normally connected to the internal speech line HL via contacts a4 to a6 and ru3 to ru5 of relays A and RU in each station. Any station can by lifting the handset and dialling a number be connected to any other station over the internal line. The central control switch elements I to IV respond operate contacts such as 12, 33 and 43 connecting earth supplied via a resistor Rf to a ring control line such as snh individual to the called station. This resistive earth provides at the called station current sufficient to operate the ringer SN at the called station. When the called station handset is lifted the connection is complete and if the handset is lifted at any other station that station can also join in. Calls to the exchange.-If at any station the handset is lifted and the exchange key AT operated, relays A and RU both respond. Contacts a 2  and a 3  of relay A connect the station to the exchange line and contact ru2 of relay R enables exchange line busy lamps Bl at each station, and operates a relay AH at the control unit looping the exchange line. Contacts ru3 to ru5 change over to prepare to connect the station to the call-back line RL. Calls from the exchange; call transfer.-A call from the exchange rings a bell AW in the control unit RSK which is at the main station RHSt. The subscriber at the main station responds lifting his handset and pressing his exchange key AT. The central control unit loops the exchange line AL at ah1 as for an outgoing call and the subscriber can talk to the caller. If during the call the subscriber presses his callback key his relay A releases while RU continues to hold and his subset is connected to the call-back line. The subscriber can then dial the number of an extension to which he wishes to make an enquiry or transfer the call and the control unit responds as for an internal call to connect earth on the ring control line such as snh individual to that extension. The earth is however connected directly to the ring control line via a contact S16 which by-passes the resistor Rf. The current is thus sufficient to operate the relay RU at the extension station in addition to the bell at that station so that when the extension responds it is connected to the call-back line. Either station may reestablish the connection to the exchange line by pressing that station exchange key and further enquiries and transfers may take place in similar fashion. The keys at each station are non-latching and the circuits they control are of the self-holding type. The main station exchange key controls an auxiliary switching element which when the main station operates its exchange key to respond to a call changes the function of the exchange key such that if it is again operated the speech line can be earthed.